It has been proposed that the high incidence of obesity in the Pima Indians may be due to genetic selection for a "thrifty gene", a gene that would predispose the Pimas to store energy very efficiently. These efficient individuals would presumably be more capable of surviving famine than lean, less efficient individuals. In order to determine whether or not such a genetic difference exists, resting metabolic rates and responses to three thermogenic stimuli (food, norepinerphrine and exercise) were measured. Since overeating obviously precedes obesity, the aforementioned parameters were measured during weight maintenance as well as during overfeeding; certain individuals were also studied during underfeeding. Thyroid kinetics studies were also performed on a group of obese Pima volunteers, since these hormones have been shown to be affected by the nutritional state of an individual. The results from this research are being compared to those obtained from lean andobese Caucasians involved in studies performed in Vermont.